Various schemes have been developed in the field of automatic transmissions to provide an overdrive ratio. The overdrive ratio is desirable as an additional ratio for the advantages of fuel economy and flexibility in the ratio coverage available. Normal solutions to this problem, as recognized in the prior art, include overdrive gear sets added in series to the transmission structure which adds greatly to the space requirements, not only in the axial sense but also in the radial sense as compared to the transmission without the overdrive ratio. In addition, many of the prior art designs require the utilization of a friction brake for the overdrive ratio and a friction clutch for the locked-up condition of the overdrive gear set, the locked-up condition being required when the transmission is operating in ratios other than overdrive. When both a friction clutch and friction brake are utilized, difficulties are encountered in shifting into and out of the overdrive ratio since a timing problem exists to insure a smooth shift from the previously established ratio into the overdrive ratio and, likewise, a smooth down-shift from the overdrive ratio to a lower drive ratio. Further, additional friction clutches as normally required in an overdrive gear set require a rather significant amount of axial space within the transmission.
To summarize the state of the art with regard to addition of overdrive ratios in automatic transmissions, the main problems are space problems, smooth engagement, and timing of the ratio changes into and out of overdrive ratio.